docs: def Stokes_number(V, Dp, D, rhop, mu):
      r'''Calculates Stokes Number for a given characteristic velocity `V`, 
      particle diameter `Dp`, characteristic diameter `D`, particle density 
      `rhop`, and fluid viscosity `mu`.
  
      .. math::
          \text{Stk} = \frac{\rho_p V D_p^2}{18\mu_f D}
  
      Parameters
      ----------
      V : float
          Characteristic velocity (often superficial), [m/s]
      Dp : float
          Particle diameter, [m]
      D : float
          Characteristic diameter (ex demister wire diameter or cyclone 
          diameter), [m]
      rhop : float
          Particle density, [kg/m^3]
      mu : float
          Fluid viscosity, [Pa*s]
      
      Returns
      -------
      Stk : float
          Stokes numer, [-]
  
      Notes
      -----
      Used in droplet impaction or collection studies.
  
      Examples
      --------
      >>> Stokes_number(V=0.9, Dp=1E-5, D=1E-3, rhop=1000, mu=1E-5)
      0.5
  
      References
      ----------
      .. [1] Rhodes, Martin J. Introduction to Particle Technology. Wiley, 2013.
      .. [2] Al-Dughaither, Abdullah S., Ahmed A. Ibrahim, and Waheed A. 
         Al-Masry. "Investigating Droplet Separation Efficiency in Wire-Mesh Mist
         Eliminators in Bubble Column." Journal of Saudi Chemical Society 14, no.
         4 (October 1, 2010): 331-39. https://doi.org/10.1016/j.jscs.2010.04.001.
      '''
      return rhop*V*(Dp*Dp)/(18.0*mu*D)